Radiation detecting element sensitivity correction method and radiation tomography device

ABSTRACT

Sensitivity correction for multiple γ radiation detectors is performed by use of sensitivity coefficients obtained through a first sensitivity coefficient calculation step for obtaining sensitivity coefficients, classified according to sensitivity factors, on the basis of coincidence counting data collected as a result of detection of γ radiation emitted from a rotated rod-shaped calibration radiation source, and through a third sensitivity coefficient calculation step for obtaining sensitivity coefficients derived from a geometrical arrangement on the basis of coincidence counting data collected in a state where arrangement of the γ radiation detectors is changed. A re-constructed image is obtained on the basis of data acquired after the sensitivity correction is finished.

TECHNICAL FIELD

This invention relates to a sensitivity correction method for aradiation detecting element that detects radiation generated from aradiopharmaceutical in a test object and a radiation tomography devicethat performs radiation tomography by detecting radiation generated fromthe radiopharmaceutical in the test object.

BACKGROUND ART

In positron emission tomography (PET) of a radiation tomography device,a sensitivity difference between detectors needs to be calibrated toobtain homogeneous sensitivity of all detectors at the time of obtaininga reconstructed image. Hereinafter, this calibration will be referred toas “sensitivity correction”. A coefficient for sensitivity correction(hereinafter referred to as “sensitivity coefficient”) is calculatedfrom actual data collected using a calibration radiation source, and amethod of calculating a sensitivity coefficient has two types.

The first method is a direct method of obtaining sensitivitycoefficients for all detector pairs. In the direct method, thesensitivity coefficients of all detector pairs are directly obtained.However, when the number of detectors is large, the number of detectorpairs becomes enormous by the square of the number of detectors.Therefore, statistical accuracy per pair is lowered, and the number ofsensitivity coefficients becomes enormous.

The next one is an “element-by-element sensitivity correction method” ofdecomposing each factor that varies sensitivity and setting a productthereof to a sensitivity coefficient of a detector pair (for example,see Patent Documents 1 and 2 and Non-Patent Documents 1 to 3). In thiselement-by-element sensitivity correction method, when a pair ofcoinciding detector rings is set to (u, v), and a pair of coincidingdetectors in a ring is set to (i, j), a sensitivity correctioncoefficient NC_(uivj) is decomposed into elements as in the followingEquation (1).NC _(uivj)=ε_(ui)×ε_(vj) ×b _(uvk) ×d _(uvrk) ×g _(uvr) ×f _(uv)  (1)

Here, In Equation (1), ε_(ui) and ε_(vj) denote sensitivities unique todetectors, d_(uvrk) denotes a crystal interference factor, f_(uv)denotes ring pair sensitivity, b_(uvk) denotes a block profile factor,g_(uvr) denotes a radial direction geometric factor, k denotes a crystalrelative position in a block, and r denotes a radial direction position.In these elements, g_(uvr), d_(uvrk), etc. are geometrically determinedelements (hereinafter referred to as “geometric factors”). In addition,ε_(ui), ε_(vj), and b_(uvk) are non-geometric factors which change overtime.

In the element-by-element sensitivity correction method, the product ofsensitivity coefficients is used for expression, and thus the number ofsensitivity coefficients is small when compared to the direct method. Inaddition, since each factor obtained by decomposing the sensitivitycoefficient is added using geometric symmetry, statistical accuracy canbe improved. However, the sensitivity coefficient is indirectly obtainedby the product of factors, and thus is an approximate numerical value.In a recent PET apparatus, the element-by-element sensitivity correctionmethod starts to be used for the purpose of high resolution,simplification of calibration and improvement of statistical accuracy.

A calibration flow of a conventional element-by-element correctionmethod is illustrated in FIG. 1 and FIG. 2 of Non-Patent Document 1.FIG. 1 is a calibration flow for calculation of a geometric factor usinga conventional low scattering calibration radiation source and FIG. 2 isa calibration flow for calculation of a non-geometric factor using aconventional uniform cylindrical calibration radiation source. Thegeometric factor which does not change over time is calculated using thelow scattering calibration radiation source, and the non-geometricfactor which changes over time is calculated using data of the uniformcylindrical calibration radiation source corrected by a coefficient ofthe geometric factor. Then, a sensitivity coefficient of a specific pairof detectors is obtained using Equation (1) shown above. With regard toa specific calibration flow, refer to Non-Patent Document 1.

CITATION LIST Patent Document

Patent Document 1: JP-A-2008-170329

Patent Document 2: WO 2010/013356

Non-Patent Document

Non-Patent Document 1: Badawi R D, Marsden P K: Developments incomponent-based normalization for 3D PET. Phys. Med. Biol. 44, 571-594,1999

Non-Patent Document 2: Badawi R D, Lodge M A, Marsden P K: Algorithmsfor calculating detector efficiency normalization coefficients for truecoincidences in 3D PET. Phys. Med. Biol. 43, 189-205, 1998

Non-Patent Document 3: Badawi R D, Ferreira N C, Kohlmyer S G, DahlbomM, Marsden P K, Lewellen T K: A comparison of normalization effects onthree whole-body cylindrical 3D PET systems. Phys. Med. Biol. 45,3253-3266, 2000

SUMMARY OF THE INVENTION Technical Problem

However, such a conventional element-by-element sensitivity correctionmethod has the following problems.

In more detail, there is a problem that a flexible PET apparatussupporting multi-modality does not correspond to the conventionalelement-by-element sensitivity correction method. The conventionalelement-by-element sensitivity correction method is valid when adetector group has a circular arrangement and a positional relation isfixed.

As illustrated in FIG. 3, the flexible PET apparatus supportingmulti-modality includes two detector heads 2A and 2B in which arcuatedetector groups are disposed, and the respective detector heads 2A and2B are close to each other as indicated by a two-dot chain line of FIG.3. As a result, a positional relation becomes variable and elliptical,and an assumption of the element-by-element sensitivity correctionmethod fails and becomes invalid (see AB at the time of non-approach andA′B at the time of approach illustrated in FIG. 3).

In addition, when the two detector heads are close to each other, in thecase of a detector pair in the same detector head, even though asensitivity coefficient is invariant, a distance between detector headsfacing each other changes, and incidence angles of a γ-ray on thedetectors are different from each other. Thus, sensitivity of ageometric factor changes in the detector heads facing each other. As aresult, artifacts occur in a reconstructed image.

The invention has been conceived in view of such circumstances, and anobject of the invention is to provide a sensitivity correction methodfor a radiation detecting element and a radiation tomography devicecapable of obtaining a reconstructed image free of artifacts even whenarrangement of a plurality of radiation detecting elements included in adetector is variant and is not circular arrangement.

Solution to Problem

The invention adopts configurations below to attain such an object.

In more detail, a sensitivity correction method according to theinvention is a sensitivity correction method for a radiation detectingelement detecting radiation generated from a radiopharmaceutical in atest object, the sensitivity correction method including a firstsensitivity coefficient calculation step of obtaining sensitivitycoefficients classified according to sensitivity factors based oncoincidence counting data collected by a plurality of radiationdetecting elements, a second sensitivity coefficient calculation step ofobtaining a sensitivity coefficient of a non-geometric factor based onthe collected coincidence counting data before change of arrangement ofthe plurality of radiation detecting elements after the firstsensitivity coefficient calculation step, and a third sensitivitycoefficient calculation step of obtaining a sensitivity coefficientderived from geometrical arrangement based on the collected coincidencecounting data in a state in which the arrangement of the plurality ofradiation detecting elements is changed after the second sensitivitycoefficient calculation step, in which sensitivities of the radiationdetecting elements are corrected using the sensitivity coefficientsobtained in the first sensitivity coefficient calculation step, thesecond sensitivity coefficient calculation step, and the thirdsensitivity coefficient calculation step.

[Effects] According to the sensitivity correction method according tothe invention, first, a sensitivity coefficient is obtained in the firstsensitivity coefficient calculation step. After the first sensitivitycoefficient calculation step, a sensitivity coefficient is obtained inthe second sensitivity coefficient calculation step before a change inarrangement of the plurality of radiation detecting elements. After thesecond sensitivity coefficient calculation step, a sensitivitycoefficient is obtained in the third sensitivity coefficient calculationstep in a state in which the arrangement of the plurality of radiationdetecting elements is changed. Specifically, in the first sensitivitycoefficient calculation step, sensitivity coefficients classifiedaccording to sensitivity factors are obtained. Even when the arrangementof the plurality of radiation detecting elements is not changed, anon-geometric factor changes over time. Therefore, a sensitivitycoefficient of a non-geometric factor is obtained in the secondsensitivity coefficient calculation step based on collected coincidencecounting data before change of the arrangement of the plurality ofradiation detecting elements. Then, in the third sensitivity coefficientcalculation step, a sensitivity coefficient derived from geometricalarrangement is obtained based on collected coincidence counting data inthe state in which in which arrangement of the plurality of radiationdetecting elements is changed. In this way, sensitivity correction ofthe radiation detecting elements is performed using the sensitivitycoefficients obtained in the first sensitivity coefficient calculationstep, the second sensitivity coefficient calculation step, and the thirdsensitivity coefficient calculation step, thereby obtaining areconstructed image based on data after the sensitivity correction. As aresult, even when arrangement of the plurality of radiation detectingelements included in the detector is variable and is not circulararrangement, it is possible to obtain a reconstructed image free ofartifacts.

In a specific example, a sensitivity coefficient corresponding to a casein which the arrangement of the plurality of radiation detectingelements is changed is represented by a product of a sensitivitycoefficient of a geometric factor obtained before the change of thearrangement and a sensitivity coefficient of a non-geometric factor withrespect to a combination in which a positional relation is invariantirrespective of the arrangement of the radiation detecting elements (forexample, the same detector head). Further, the sensitivity coefficientis represented by a product of a sensitivity coefficient of a geometricfactor obtained after the change of the arrangement and a sensitivitycoefficient of a non-geometric factor with respect to a combinationgeometrically affected by a positional relation of the radiationdetecting elements (for example, detector heads facing each other).

In addition, it is preferable to obtain the correction coefficient froma ratio of coincidence counting data collected at the same time usingthe same calibration radiation source before and after the change of thearrangement of the plurality of radiation detecting elements. In thisway, it is possible to correct a difference generated due to the changeof the arrangement at the time of using the same calibration radiationsource. Consequently, for example, in a case in which after anothercalibration radiation source (the former: first calibration radiationsource) is used before the change of the arrangement, the samecalibration radiation source (the latter: second calibration radiationsource) is used before and after the change of the arrangement of theplurality of radiation detecting elements, it is possible to correct adifference between the former before the change of the arrangement andthe latter after the change of the arrangement by multiplying theobtained correction coefficient by the right side of above Equation (1)in the former.

In addition, coincidence counting data collected after the change of thearrangement of the plurality of radiation detecting elements correspondsto a combination geometrically affected at least by a positionalrelation of the radiation detecting elements. That is, it is possible toobtain sensitivity coefficient data with respect to the combinationgeometrically affected by the positional relation of the radiationdetecting elements (for example, the detector heads facing each other)based on the coincidence counting data collected after the change of thearrangement.

In addition, in the first sensitivity coefficient calculation step, itis preferable to obtain the sensitivity coefficient using a calibrationradiation source below. In more detail, when a rod-shaped radiationsource for calibration rotatable along the plurality of radiationdetecting elements is used, the rod-shaped radiation source forcalibration is rotated along the plurality of radiation detectingelements, thereby being approximated to a ring radiation source forcalibration. In addition, it is possible to use a ring radiation sourcefor calibration provided along the plurality of radiation detectingelements. In addition, in a case of using a plate radiation source forcalibration rotatable along the plurality of radiation detectingelements, the plate radiation source for calibration may be rotatedalong the plurality of radiation detecting elements. In this way, in thefirst sensitivity coefficient calculation step, sensitivity coefficientsclassified according to sensitivity factors are obtained based oncollected coincidence counting data by detecting radiation from therotated rod-shaped radiation source for calibration (that is, therod-shaped radiation source for calibration approximated to the ringradiation source for calibration), the ring radiation source forcalibration, or the rotated plate radiation source for calibration.

In addition, in the second sensitivity coefficient calculation step, itis preferable to obtain a sensitivity coefficient using a calibrationradiation source below. In more detail, in the second sensitivitycoefficient calculation step, it is possible to obtain a sensitivitycoefficient of a non-geometric factor which changes over time based oncoincidence counting data collected by detecting radiation from acylindrical radiation source for calibration imitating an actual testobject before the change of the arrangement of the plurality ofradiation detecting elements.

In addition, in a case in which a plurality of detector heads, each ofwhich includes the plurality of radiation detecting elements, isincluded, and the respective detector heads are configured such that adistance between the respective detector heads can be changed, when thearrangement of the plurality of radiation detecting elements is changedby bringing the respective detector heads close to each other, it ispreferable to obtain a sensitivity coefficient using a calibrationradiation source below. In more detail, when the respective detectorheads are brought close to each other, there is concern that therod-shaped radiation source for calibration rotatable along theplurality of radiation detecting elements or the ring radiation sourcefor calibration provided along the plurality of radiation detectingelements may interfere with the respective detector heads. Therefore, aplate radiation source for calibration installed at a center of therespective detector heads is used, or a rod-shaped radiation source forcalibration that can reciprocate along a central axis of the respectivedetector heads is used. When the rod-shaped radiation source forcalibration is used, the rod-shaped radiation source for calibration isreciprocated along the central axis of the respective detector heads,thereby being approximated to the plate radiation source forcalibration. In the third sensitivity coefficient calculation step, asensitivity coefficient derived from geometrical arrangement is obtainedbased on coincidence counting data collected by detecting radiation fromthe plate radiation source for calibration or the reciprocatedrod-shaped radiation source for calibration (that is, the rod-shapedradiation source for calibration approximated to the plate radiationsource for calibration) in a state in which the respective detectorheads are brought close to each other. When such a plate radiationsource for calibration or a reciprocated rod-shaped radiation source forcalibration (that is, a rod-shaped radiation source for calibrationapproximated to the plate radiation source for calibration) is used, itis possible to prevent interference between the calibration radiationsource and each detector head even when the respective detector headsare brought close to each other.

In addition, a radiation tomography device according to the invention isa radiation tomography device performing radiation tomography bydetecting radiation generated from a radiopharmaceutical in a testobject, the radiation tomography device including a plurality ofdetector heads, each of which has a plurality of radiation detectingelements, the respective detector heads being configured such that adistance between the respective detector heads is changeable, andarithmetic means that performs an arithmetic operation, in which thearithmetic means performs a first sensitivity coefficient calculationstep of obtaining sensitivity coefficients classified according tosensitivity factors based on coincidence counting data collected by theplurality of radiation detecting elements, a second sensitivitycoefficient calculation step of obtaining a sensitivity coefficient of anon-geometric factor based on the collected coincidence counting databefore change of arrangement of the plurality of radiation detectingelements after the first sensitivity coefficient calculation step, and athird sensitivity coefficient calculation step of obtaining asensitivity coefficient derived from geometrical arrangement based onthe collected coincidence counting data in a state in which thearrangement of the plurality of radiation detecting elements is changedafter the second sensitivity coefficient calculation step, andsensitivities of the radiation detecting elements are corrected usingthe sensitivity coefficients obtained in the first sensitivitycoefficient calculation step, the second sensitivity coefficientcalculation step, and the third sensitivity coefficient calculationstep.

[Effects] The radiation tomography device according to the inventionincludes the arithmetic means that performs the first sensitivitycoefficient calculation step, the second sensitivity coefficientcalculation step, and the third sensitivity coefficient calculation stepas described in the sensitivity correction method according to theinvention. As described in the sensitivity correction method accordingto the invention, a reconstructed image is obtained based on data aftersensitivity correction by correcting sensitivities of the radiationdetecting elements using the sensitivity coefficients obtained in thefirst sensitivity coefficient calculation step, the second sensitivitycoefficient calculation step, and the third sensitivity coefficientcalculation step. As a result, even when arrangement of the plurality ofradiation detecting elements included in the detector is variable and isnot circular arrangement, it is possible to obtain a reconstructed imagefree of artifacts.

Advantageous Effects of the Invention

According to a sensitivity correction method for a radiation detectingelement and a radiation tomography device according to the invention, areconstructed image is obtained based on data after sensitivitycorrection by correcting sensitivities of the radiation detectingelements using the sensitivity coefficients obtained in the firstsensitivity coefficient calculation step (of obtaining sensitivitycoefficients classified according to sensitivity factors based oncoincidence counting data collected by the plurality of radiationdetecting elements), the second sensitivity coefficient calculation step(of obtaining a sensitivity coefficient of a non-geometric factor basedon the collected coincidence counting data before change of arrangementof the plurality of radiation detecting elements after the firstsensitivity coefficient calculation step), and the third sensitivitycoefficient calculation step (of obtaining a sensitivity coefficientderived from geometrical arrangement based on the collected coincidencecounting data in a state in which the arrangement of the plurality ofradiation detecting elements is changed after the second sensitivitycoefficient calculation step). As a result, even when arrangement of theplurality of radiation detecting elements included in the detector isvariable and is not circular arrangement, it is possible to obtain areconstructed image free of artifacts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view and a block diagram illustratingone mode of a detector head of a flexible PET apparatus supportingmulti-modality according to an embodiment.

FIG. 2 is a schematic perspective view of a γ radiation detector.

FIG. 3(a) is a schematic view of the detector head seen from a depthdirection, and FIG. 3(b) is a schematic view seen from a horizontaldirection orthogonal to the depth direction.

FIGS. 4(a) to 4(c) are schematic views of detector arrangement and typesof the calibration radiation source, FIG. 4(a) is a schematic view of aring radiation source at the time of non-approach, FIG. 4(b) is aschematic view of a cylindrical radiation source at the time ofnon-approach, FIG. 4(c) is a schematic view of a plate radiation sourceat the time of approach.

FIG. 5 is a flowchart illustrating a flow of a processing procedure ofan element-by-element sensitivity correction method according to anembodiment.

FIG. 6 is a schematic view at the time of rotating a rod-shapedradiation source, which is rotatable along the γ radiation detector ofthe detector head, along the γ radiation detector of the detector head.

FIGS. 7(a) to 7(c) are schematic views of detector arrangement and typesof a calibration radiation source according to modifications.

EMBODIMENTS

Hereinafter, embodiments of the invention will be described withreference to drawings.

FIG. 1 is a schematic perspective view and a block diagram illustratingone mode of a detector head of a flexible PET apparatus supportingmulti-modality according to an embodiment. FIG. 2 is a schematicperspective view of a γ radiation detector. FIG. 3(a) is a schematicview of the detector head viewed in a depth direction, and FIG. 3(b) isa schematic view viewed in a horizontal direction orthogonal to thedepth direction.

As illustrated in FIG. 1, the flexible PET apparatus supportingmulti-modality (hereinafter simply referred to as a “PET apparatus”) 1includes detector heads 2A and 2B. As illustrated in FIG. 2, onedetector head includes a total of 18×3 detector blocks 31 in whicheighteen detector blocks 31 (γ radiation detectors 3) are arranged in anarcuate shape and three detector blocks 31 are arranged in the depthdirection z (see FIG. 1). Here, 16×16 detector crystals arranged invertical and horizontal directions are stacked in four layers in one ofthe detector blocks 31. A specific configuration of the γ radiationdetector 3 other than the detector block 31 will be described below inFIG. 2. The flexible PET apparatus supporting multi-modality (PETapparatus) 1 corresponds to a radiation tomography device in theinvention, the detector heads 2A and 2B correspond to a detector head inthe invention, and the γ radiation detector 3 corresponds to a radiationdetecting element in the invention.

As illustrated in FIG. 1, the detector heads 2A and 2B are configured toface each other. The detector heads 2A and 2B are normally located on acircumference of a 48-sided polygon. The number of the detector blocks31 (γ radiation detectors 3) in a cross section of the two the detectorheads 2A and 2B is 36 in total (18×2 in the arcuate shape). However, dueto the 48-sided polygon, a gap region 4 corresponding to six is presentin each of right and left sides. As described in the section “TechnicalProblem”, in FIG. 3, the detector heads 2A and 2B may be brought closeto each other while the detector heads 2A and 2B face each other. Inthis manner, the respective detector heads 2A and 2B are configured suchthat a distance between the respective detector heads 2A and 2B can bechanged.

Besides, the PET apparatus 1 includes a coincidence circuit 5 and anarithmetic circuit 6. A specific function of the arithmetic circuit 6will be described below with reference to FIG. 4. The arithmetic circuit6 corresponds to arithmetic means in the invention.

When a radiopharmaceutical is administered to a test object (notillustrated) to which a radiopharmaceutical has been administered, twoγ-rays are generated since a positron of a positron emission type RIdisappears. The coincidence circuit 5 checks the position of thedetector blocks 31 (see FIG. 2) and incidence timing of the γ-rays, anddetermines fed image information to be appropriate data (coincidencecounting data) only when the γ-rays are simultaneously incident on thetwo detector blocks 31 on both sides of the test object. When the γ-raysare incident only on one of the detector blocks 31, the coincidencecircuit 5 rejects the incidence. That is, the coincidence circuit 5detects that the γ-rays are simultaneously observed (that is,coincidence) in the two γ radiation detectors 3 based on theabove-described electric signal. The image information (coincidencecounting data) collected by the coincidence circuit 5 is sent to thearithmetic circuit 6.

As illustrated in FIG. 2, the γ radiation detector 3 includes thedetector blocks 31, a light guide 32 optically coupled to the detectorblocks 31, and a photo multiplier tube (PMT) 33 optically coupled to thelight guide 32. Each detector crystal included in the detector blocks 31emits light in response to incidence of a γ-ray to convert from theγ-ray into light. Through this conversion, the detector crystal detectsthe γ-ray. Light emitted from the detector crystal is sufficientlydiffused by the detector blocks 31 and input to the photo multipliertube (hereinafter simply abbreviated to “PMT”) 33 through the lightguide 32. The PMT 33 multiplies the light converted by the detectorblocks 31 and converts the multiplied light into an electric signal. Theelectrical signal is sent to the coincidence circuit 5 (see FIG. 1) asimage information (coincidence counting data) as described above.

In addition, as illustrated in FIG. 2, the γ radiation detector 3 is adepth of interaction (DOI) detector including a plurality of layers inthe depth direction. Even though FIG. 2 illustrates a four-layer DOIdetector, the number of layers is not particularly limited as long as aplurality of layers is provided. In addition, the number of verticallyand horizontally arranged detector crystals is not particularly limitedas long as a plurality of detector crystals is provided.

Here, the DOI detector is formed by stacking the respective detectorcrystals in a depth direction of a radiation, and coordinate informationof the depth direction (DOI) in which an interaction occurs and alateral direction (a direction parallel to an incident plane) isobtained by calculation of a center of gravity. It is possible tofurther improve spatial resolution in the depth direction using the DOIdetector. Therefore, the number of layers of the DOI detectorcorresponds to the number of layers of the detector crystals stacked inthe depth direction.

Next, a specific configuration of the arithmetic circuit 6 will bedescribed with reference to FIG. 4 to FIG. 6. FIGS. 4(a) to 4(c) areschematic views of detector arrangement and types of the calibrationradiation source, FIG. 4(a) is a schematic view of a ring radiationsource at the time of non-approach, FIG. 4(b) is a schematic view of acylindrical radiation source at the time of non-approach, FIG. 4(c) is aschematic view of a plate radiation source at the time of approach. FIG.5 is a flowchart illustrating a flow of a processing procedure of anelement-by-element sensitivity correction method according to anembodiment. FIG. 6 is a schematic view at the time of rotating arod-shaped radiation source, which is rotatable along the γ radiationdetector of the detector head, along the γ radiation detector of thedetector head.

Reference symbol S_(R) of FIG. 4(a) denotes a ring radiation source forcalibration (hereinafter simply abbreviated to a “ring radiationsource”), reference symbol S_(C) of FIG. 4(b) denotes a cylindricalradiation source for calibration (hereinafter simply abbreviated to a“cylindrical radiation source”), and reference symbol S_(P) of FIG. 4(c)denotes a plate radiation source for calibration (hereinafter simplyabbreviated to a “plate radiation source”). In the present embodiment,as illustrated in FIG. 6, a rod-shaped radiation source for calibration(hereinafter simply abbreviated to a “rod-shaped radiation source”)S_(L) rotatable along the γ radiation detectors 3 (see FIG. 2) (that is,the radiation detecting elements) of the detector heads 2A and 2B isused, and the rod-shaped radiation source S_(L) is rotated along the γradiation detectors 3 of the detector heads 2A and 2B, thereby beingapproximated to the ring radiation source S_(R) illustrated in FIG.4(a). Hereinafter, the rod-shaped radiation source S_(L) illustrated inFIG. 6 will be described as the ring radiation source S_(R) illustratedin FIG. 4(a). A thickness of the ring radiation source S_(R) is about 5mm, a diameter of the cylindrical radiation source S_(C) is about 15 cm,a thickness of the plate radiation source S_(P) is about 5 mm, and adiameter of the rod-shaped radiation source S_(L) is about 15 cm.

In addition, in the present embodiment, three types are prepared anddata of each of the types is collected. Here, the three types correspondto the ring radiation source S_(R) approximated to a ring by rotatingthe rod-shaped radiation source S_(L) (see FIG. 6) at the time ofnon-approach illustrated in FIG. 4(a), the uniform cylindrical radiationsource S_(C) installed at a center of the apparatus (the detector heads2A and 2B) at the time of non-approach illustrated in FIG. 4(b), and theplate radiation source S_(P) installed at the center of the apparatus(the detector heads 2A and 2B) at the time of approach illustrated inFIG. 4(c). The calibration radiation source is made of acrylic resin,and a uniform concentration of radioactive solution is sealed inside.

In addition, in FIG. 5, a transaxial block profile factor b_(uvk)corresponds to a “Transaxial Block Profile Factor”, a crystalinterference factor d_(uvrk) corresponds to a “Crystal InterferenceFactor”, a block profile factor b_(uvk) on a body axis cross section(body axis transverse plane) corresponds to an “Axial Block ProfileFactor”, a ring pair sensitivity f_(uv) corresponds to a “Ring PairFactor”, sensitivities ε_(ui) and ε_(vj) unique to the detectorscorrespond to a “Crystal Efficiency Factor”, a crystal interferencefactor d_(Plate) at the time of the plate radiation source correspondsto a “Crystal Interference (Plate) Factor”, and a ring pair sensitivityf_(Plate) at the time of the plate radiation source corresponds to a“Ring Pair (Plate) Factor”.

(Step S1)

As illustrated in the flowchart of FIG. 5, first, the crystalinterference factor d_(uvrk) (written as “Crystal Interference Factor”)of the geometric factor that does not change overtime is calculatedusing coincidence counting data collected by the ring radiation sourceS_(R) illustrated in FIG. 4(a). It is preferable to apply path lengthstandardization (written as “Apply”) before the crystal interferencefactor d_(uvrk) (Crystal Interference Factor) of the geometric factor iscalculated.

The path length standardization refers to standardization of coincidencecounting data on a predetermined path using a length in which the pathpasses through the radiation source. A difference in length intersectingthe radiation source between predetermined paths is corrected in apseudo manner by performing the path length standardization. With regardto specific path length standardization, refer to Patent Document 1:JP-A-2008-170329.

Further, solid angle correction for correct a detector distance isperformed. Specifically, a crystal interference factor d_(uvrk) (CrystalInterference Factor) of a certain line of response (LOR) is divided bythe square of a detector distance of the LOR. A coefficient of thecrystal interference factor d_(uvrk) (Crystal Interference Factor)obtained as a result thereof is output as a table. Here, the LOR refersto a virtual straight line connecting two coinciding detectors. Step S1corresponds to a first sensitivity coefficient calculation step in theinvention.

(Step S2)

Subsequently, the sensitivities ε_(ui) and ε_(vj) unique to thedetectors (written as “Crystal Efficiency Factor”) of the non-geometricfactor that changes over time are calculated in order using coincidencecounting data of the cylindrical radiation source S_(C) illustrated inFIG. 4(b) subjected to correction of the geometric factor by the crystalinterference factor d_(uvrk) (Crystal Interference Factor). In thisinstance, the block profile factor b_(uvk) (written as “Axial BlockProfile Factor”) on the body axis cross section (body axis transverseplane) and the transaxial block profile factor b_(uvk) (written as“Transaxial Block Profile Factor”) are calculated, and correction isperformed on coincidence counting data of the plate radiation sourceS_(P) illustrated in FIG. 4(c) in step S3.

In addition, the ring pair sensitivity f_(uv) (written as “Ring PairFactor”) is calculated. Each factor obtained by calculation is output asa table. Step S2 corresponds to a second sensitivity coefficientcalculation step in the invention.

(Step S3)

Finally, coincidence counting data of the plate radiation source S_(P)illustrated in FIG. 4(c) is used to calculate only the crystalinterference factor d_(Plate) (written as “Crystal Interference (Plate)Factor”) at the time of the plate radiation source affected by change ingeometrical arrangement of the detectors due to proximity and the ringpair sensitivity f_(Plate) (written as “Ring Pair (Plate) Factor”) atthe time of the plate radiation source.

In other words, since a positional relation of a combination of the samedetector heads is invariant regardless of the arrangement of the γradiation detectors 3 (radiation detecting elements), the crystalinterference factor d_(uvrk) (Crystal Interference Factor) and the ringpair sensitivity f_(uv) (Ring Pair Factor) are not calculated in stepS3. On the other hand, since a combination of detector heads facing eachother is geometrically affected by a positional relation of the γradiation detectors 3 (radiation detecting elements), the crystalinterference factor d_(Plate) (Crystal Interference (Plate) Factor) atthe time of the plate radiation source and the ring pair sensitivityf_(Plate) (Ring Pair (Plate) Factor) at the time of the plate radiationsource are calculated.

Similarly to step S1, it is preferable to apply path lengthstandardization before calculating these factors. A solid angle iscorrected at coordinates at the time of approach with respect to thecrystal interference factor d_(Plate) (Crystal Interference (Plate)Factor) at the time of the plate radiation source. Coefficients of thecrystal interference factor d_(Plate) (Crystal Interference (Plate)Factor) at the time of the plate radiation source and the ring pairsensitivity f_(Plate) (Ring Pair (Plate) Factor) at the time of theplate radiation source obtained by calculation are output as a table.Step S3 corresponds to a third sensitivity coefficient calculation stepin the invention.

With regard to specific calculation of each factor, refer to Non-PatentDocument 1. The flexible PET apparatus uses a fan-sum method since a gapis present between the detector heads. With regard to specificcalculation of the transaxial block profile factor b_(uvk) (TransaxialBlock Profile Factor) or the ring pair sensitivity f_(uv) (Ring PairFactor) using the fan-sum method, refer to Patent Document 2: WO2010/013356 A.

An equation for obtaining a correction coefficient α for correcting adifference between the plate radiation source at the time of approachand the ring radiation source at the time of non-approach is representedby the following Equation (2).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\{\frac{N_{near}}{N_{normal}} = {\alpha\frac{1/L_{near}^{2}}{1/L_{normal}^{2}}}} & (2)\end{matrix}$

In the above Equation (2), N_(near) denotes coincidence counting data(count value) collected using the plate radiation source at the time ofapproach, N_(normal) denotes coincidence counting data (count value)collected using the plate radiation source at the time of non-approach,L_(near) denotes a detector distance at the time of approach, andL_(normal) denotes a detector distance at the time of non-approach. Inthis way, a difference between the plate radiation source and the ringradiation source corresponding to a right side of the above Equation (2)is represented by the product of a geometrical factor (in this case,L_(near) and L_(normal)) represented by a detector distance and anothercorrection coefficient α. A left side of the above Equation (2) isexperimentally obtained by collecting coincidence counting data at thesame time using the same plate radiation source at the time ofapproach/at the time of non-approach. In the present embodiment, a shortside transformed into an ellipse is adopted as the inter-detectordistance L_(near).

According to the sensitivity correction method according to the presentembodiment, first, a sensitivity coefficient is obtained in the firstsensitivity coefficient calculation step (step S1). After the firstsensitivity coefficient calculation step (step S1), a sensitivitycoefficient is obtained in the second sensitivity coefficientcalculation step (step S2) before a change in arrangement of a pluralityof radiation detecting elements (γ radiation detectors in the presentembodiment) (at the time of non-approach in the present embodiment).After the second sensitivity coefficient calculation step (step S2), asensitivity coefficient is obtained in the third sensitivity coefficientcalculation step (step S3) in a state in which arrangement of theplurality of radiation detecting elements is changed (in the presentembodiment, the detector heads 2A and 2B are close to each other).Specifically, in the first sensitivity coefficient calculation step(step S1), sensitivity coefficients classified according to sensitivityfactors are obtained. Even when the arrangement of the plurality ofradiation detecting elements (γ radiation detectors) is not changed, anon-geometric factor changes over time. Therefore, a sensitivitycoefficient of a non-geometric factor is obtained in the secondsensitivity coefficient calculation step (step S2) based on collectedcoincidence counting data before change of the arrangement of theplurality of radiation detecting elements (γ radiation detectors) (atthe time of non-approach). Then, in the third sensitivity coefficientcalculation step (step S3), a sensitivity coefficient derived fromgeometrical arrangement is obtained based on collected coincidencecounting data in the state in which in which arrangement of theplurality of radiation detecting elements is changed (the detector heads2A and 2B are close to each other). In this way, sensitivity correctionof the radiation detecting elements (γ radiation detectors) is performedusing the sensitivity coefficients obtained in the first sensitivitycoefficient calculation step (step S1), the second sensitivitycoefficient calculation step (step S2), and the third sensitivitycoefficient calculation step (step S3), thereby obtaining areconstructed image based on data after the sensitivity correction. As aresult, even when arrangement of the plurality of radiation detectingelements (γ radiation detectors) included in the detector is variableand is not circular arrangement, it is possible to obtain areconstructed image free of artifacts.

In the present embodiment, a sensitivity coefficient corresponding tothe case in which arrangement of the plurality of radiation detectingelements is changed (the detector heads 2A and 2B are close to eachother) is represented by the product of a sensitivity coefficient of ageometric factor obtained before change of arrangement (at the time ofnon-approach in the present embodiment) and a sensitivity coefficient ofa non-geometric factor with regard to a combination in which apositional relation is invariant regardless of arrangement of theradiation detecting elements (γ radiation detectors) (the same detectorhead in the present embodiment). Further, the sensitivity coefficient isrepresented by the product of a sensitivity coefficient of a geometricfactor obtained after change of arrangement (at the time of approach inthe present embodiment) and a sensitivity coefficient of a non-geometricfactor with regard to a combination geometrically affected by thepositional relation of the radiation detecting elements (γ radiationdetectors) (detector heads facing each other in the present embodiment).

When a sensitivity coefficient is obtained in a case in which thedetector heads 2A and 2B are close to each other, the PET apparatus 1(for example, a central processing unit (CPU)) determines whether adetector pair corresponds to the same detector head or the detectorheads facing each other. In the case of the detector heads facing eachother, switching to the sensitivity coefficient at the time of approachobtained from the plate radiation source S_(P) illustrated in FIG. 4(c)is performed.

It is preferable to obtain the correction coefficient α from a ratio ofcoincidence counting data (the left side of the above Equation (2))collected at the same time using the same calibration radiation source(the same plate radiation source in the present embodiment) before andafter change of arrangement (at the time of approach/at the time ofnon-approach) of the plurality of radiation detecting elements (γradiation detectors) as in the above Equation (2). Consequently, forexample, in a case in which after another calibration radiation source(the former: first calibration radiation source, ring radiation sourcein the present embodiment) is used before change of arrangement, thesame calibration radiation source (the latter: second calibrationradiation source, plate radiation source in the present embodiment) isused before and after change of arrangement (at the time of approach/atthe time of non-approach) of the plurality of radiation detectingelements (γ radiation detectors), it is possible to correct a differencebetween the former (the ring radiation source at the time ofnon-approach) before change of arrangement and the latter (the plateradiation source at the time of approach) after change of arrangement bymultiplying the obtained correction coefficient α by the right side ofthe above Equation (1) in the former (ring radiation source).

In addition, coincidence counting data collected after change ofarrangement (at the time of approach) of the plurality of radiationdetecting elements (γ radiation detectors) corresponds to a combinationgeometrically affected at least by a positional relation of theradiation detecting elements (γ radiation detectors). That is, it ispossible to obtain sensitivity coefficient data with respect to thecombination geometrically affected by the positional relation of theradiation detecting elements (γ radiation detectors) (the detector headsfacing each other in the present embodiment) based on the coincidencecounting data collected after change of arrangement (at the time ofapproach).

In addition, in the first sensitivity coefficient calculation step (stepS1), it is preferable to obtain the sensitivity coefficient using acalibration radiation source below. In more detail, when the rod-shapedradiation source S_(L) (see FIG. 6) rotatable along the plurality ofradiation detecting elements (γ radiation detectors) is used as in thepresent embodiment, the rod-shaped radiation source S_(L) is rotatedalong the plurality of radiation detecting elements (γ radiationdetectors), thereby being approximated to the ring radiation sourceS_(R) illustrated in FIG. 4(a). In the first sensitivity coefficientcalculation step (step S1), sensitivity coefficients classifiedaccording to sensitivity factors are obtained based on coincidencecounting data collected by detecting radiation (γ-ray in the presentembodiment) from the rotated rod-shaped radiation source S_(L) (that is,the rod-shaped radiation source S_(L) approximated to the ring radiationsource S_(R)).

In addition, in the second sensitivity coefficient calculation step(step S2), it is preferable to obtain a sensitivity coefficient using acalibration radiation source below. In more detail, in the secondsensitivity coefficient calculation step (step S2), it is possible toobtain a sensitivity coefficient of a non-geometric factor which changesover time based on coincidence counting data collected by detectingradiation (γ-ray) from the cylindrical radiation source S_(C) imitatingan actual test object before change of arrangement of the plurality ofradiation detecting elements (γ radiation detectors) (at the time ofnon-approach).

In addition, in a case in which a plurality of (two in the presentembodiment) detector heads, each of which includes the plurality ofradiation detecting elements (γ radiation detectors), is included, andthe respective detector heads are configured such that a distancebetween the respective detector heads can be changed, when arrangementof the plurality of radiation detecting elements (γ radiation detectors)is changed by bringing the respective detector heads close to eachother, it is preferable to obtain a sensitivity coefficient using acalibration radiation source below. In more detail, when the respectivedetector heads are brought close to each other, there is concern thatthe rod-shaped radiation source rotatable along the plurality ofradiation detecting elements (γ radiation detectors) or the ringradiation source provided along the plurality of radiation detectingelements (γ radiation detectors) may interfere with the respectivedetector heads. Therefore, as in the present embodiment, the plateradiation source S_(P) illustrated in FIG. 4(c) disposed at a center ofthe respective detector heads is used as in the present embodiment. Inthe third sensitivity coefficient calculation step (step S3), asensitivity coefficient derived from geometrical arrangement is obtainedbased on coincidence counting data collected by detecting radiation(γ-ray) from the plate radiation source S_(P) in a state in which therespective detector heads are brought close to each other. When such aplate radiation source S_(P) is used, it is possible to preventinterference between the calibration radiation source and each detectorhead even when the respective detector heads are brought close to eachother.

In addition, the radiation tomography device (the flexible PET apparatussupporting multi-modality 1) according to the present embodimentincludes the arithmetic means (the arithmetic circuit 6 in FIG. 1) thatperforms the first sensitivity coefficient calculation step (step S1),the second sensitivity coefficient calculation step (step S2), and thethird sensitivity coefficient calculation step (step S3) described inthe sensitivity correction method according to the present embodiment.As described in the sensitivity correction method according to thepresent embodiment, a reconstructed image is obtained based on dataafter sensitivity correction by correcting sensitivities of theradiation detecting elements (γ radiation detectors) using thesensitivity coefficients obtained in the first sensitivity coefficientcalculation step (step S1), the second sensitivity coefficientcalculation step (step S2), and the third sensitivity coefficientcalculation step (step S3). As a result, even when arrangement of theradiation detecting elements (γ radiation detectors) included in thedetector is variable and is not circular arrangement, it is possible toobtain a reconstructed image free of artifacts.

[Reconstruction Result]

A cylindrical radiation source having a uniform radiation concentrationof 300 mm in diameter was installed at the center of the apparatus(detector heads 2A and 2B) in the vicinity of a diameter of 778 mm to adiameter of 250 mm of the apparatus (detector heads 2A and 2B) tocollect coincidence counting data. In a reconstruction result ofincorporating a conventional element-by-element sensitivity correctionmethod using a geometric factor of a ring radiation source and anon-geometric factor of a cylindrical radiation source, a geometricfactor of non-approach is used, and thus artifacts occur in areconstructed image. It was confirmed that artifacts are not present andan image is improved in a reconstruction result of inputting anon-geometric factor of a cylindrical radiation source, a geometricfactor switched to an origin of a ring radiation source in an eventwithin the same detector head and to an origin of a plate radiationsource in an event within detector heads facing each other, and thecorrection coefficient α.

The invention is not limited to the above embodiment, and may bemodified as follows.

(1) While radiation corresponds to the γ-ray in the above-describedembodiment, it is possible to adopt radiation such as an α-ray, a β-ray,etc.

(2) While the DOI detector is adopted in the above-described embodiment,it is possible to apply a radiation detector which does not discriminatea depth direction.

(3) While the two detector heads 2A and 2B (see FIG. 2 to FIG. 4 andFIG. 6) have been described in the above-described embodiment, thenumber of detector heads is not limited to two. In a structure includinga plurality of detector heads, each of which has a plurality ofradiation detecting elements (γ radiation detectors in the embodiment),and the respective detector heads are configured such that a distancebetween the respective detector heads can be changed, three or moredetector heads may be adopted.

(4) In the above-described embodiment, the rod-shaped radiation sourceS_(L) (see FIG. 6) rotatable along the plurality of radiation detectingelements (γ radiation detectors in the embodiment) is used, and therod-shaped radiation source S_(L) is rotated along the plurality ofradiation detecting elements (γ radiation detectors in the embodiment),thereby being approximated to the ring radiation source S_(R)illustrated in FIG. 4(a). However, the ring radiation source S_(R)illustrated in FIG. 4(a) or the plate radiation source S_(P) illustratedin FIG. 7(a) may be rotated along the plurality of radiation detectingelements (γ radiation detectors). That is, it is possible to use thering radiation source S_(R) provided along the plurality of radiationdetecting elements (γ radiation detectors) as illustrated in FIG. 4(a).In addition, in the case of using the plate radiation source S_(P)rotatable along the plurality of radiation detecting elements (γradiation detectors) as illustrated in FIG. 7(a), the plate radiationsource S_(P) may be rotated along the plurality of radiation detectingelements (γ radiation detectors).

(5) In the above-described embodiment, the plate radiation source S_(P)illustrated in FIG. 4(c) installed at the center of the respectivedetector heads is used as the plate radiation source for calibration atthe time of approach. However, it is possible to use the rod-shapedradiation source S_(L) that can reciprocate along a central axis of eachdetector head illustrated in FIG. 7(b). That is, in the case of usingthe rod-shaped radiation source S_(L) that can reciprocate along thecentral axis of each detector head illustrated in FIG. 7(b), therod-shaped radiation source S_(L) is reciprocated along the central axisof each detector head, thereby being approximated to the plate radiationsource S_(P).

(6) While the ring radiation source S_(R) having a perfect circle shapeis used as illustrated in FIG. 4(a) in the above-described embodiment,it is possible to use a ring radiation source for calibration having ashape conforming to deformed detector arrangement (here, the ringradiation source S_(R) having an elliptical shape illustrated in FIG.7(c)). Alternatively, a rod-shaped radiation source rotatable in anelliptical orbit along the plurality of radiation detecting elements (γradiation detectors in the embodiment) may be used, and the rod-shapedradiation source may be rotated in the elliptical orbit along theplurality of radiation detecting elements (γ radiation detectors),thereby being approximated to the ring radiation source S_(R) having theelliptical shape illustrated in FIG. 7(c).

(7) In the above-described embodiment, a time before change ofarrangement of the plurality of radiation detecting elements (γradiation detectors in the embodiment) corresponds to the time ofnon-approach, and a time after the change of arrangement corresponds tothe time of approach. However, the reverse thereof is allowed. That is,the time before the change of arrangement may correspond to the time ofapproach, and the time after the change of arrangement may correspond tothe time of non-approach. Therefore, in the first sensitivitycoefficient calculation step (step S1), sensitivity coefficientclassified according to sensitivity factors are obtained based oncoincidence counting data collected at the time of approach using thering radiation source S_(R) having the elliptical shape illustrated inFIG. 7(c). Further, in the third sensitivity coefficient calculationstep (step S3), a sensitivity coefficient derived from geometricalarrangement is obtained based on coincidence counting data collected atthe time of non-approach.

(8) The shape of the calibration radiation source in the firstsensitivity coefficient calculation step (step S1), the secondsensitivity coefficient calculation step (step S2), and the thirdsensitivity coefficient calculation step (step S3) is not particularlylimited. It is sufficient to use a calibration radiation source having ashape conforming to the detector arrangement.

REFERENCE SIGNS LIST

1 Flexible PET apparatus supporting multi-modality (PET apparatus)

2A, 2B Detector head

3 γ radiation detector

S_(R) Ring radiation source for calibration (ring radiation source)

S_(C) Cylindrical radiation source for calibration (cylindricalradiation source)

S_(P) Plate radiation source for calibration (plate radiation source)

S_(L) Rod-shaped radiation source for calibration (rod-shaped radiationsource)

α Correction coefficient

The invention claimed is:
 1. A sensitivity correction method for aradiation detecting element detecting radiation generated from aradiopharmaceutical in a test object, the sensitivity correction methodcomprising: a first sensitivity coefficient calculation step ofobtaining sensitivity coefficients classified according to sensitivityfactors based on coincidence counting data collected by a plurality ofradiation detecting elements; a second sensitivity coefficientcalculation step of obtaining a sensitivity coefficient of anon-geometric factor based on the collected coincidence counting databefore change of arrangement of the plurality of radiation detectingelements after the first sensitivity coefficient calculation step; and athird sensitivity coefficient calculation step of obtaining asensitivity coefficient derived from geometrical arrangement based onthe collected coincidence counting data in a state in which thearrangement of the plurality of radiation detecting elements is changedafter the second sensitivity coefficient calculation step, whereinsensitivities of the radiation detecting elements are corrected usingthe sensitivity coefficients obtained in the first sensitivitycoefficient calculation step, the second sensitivity coefficientcalculation step, and the third sensitivity coefficient calculationstep.
 2. The sensitivity correction method according to claim 1, whereina sensitivity coefficient corresponding to a case in which thearrangement of the plurality of radiation detecting elements is changedis represented by a product of a sensitivity coefficient of a geometricfactor obtained before the change of the arrangement and a sensitivitycoefficient of a non-geometric factor with respect to a combination inwhich a positional relation is invariant irrespective of the arrangementof the radiation detecting elements and is represented by a product of asensitivity coefficient of a geometric factor obtained after the changeof the arrangement and a sensitivity coefficient of a non-geometricfactor with respect to a combination geometrically affected by apositional relation of the radiation detecting elements.
 3. Thesensitivity correction method according to claim 1, wherein a correctioncoefficient is obtained from a ratio of coincidence counting datacollected at the same time using the same calibration radiation sourcebefore and after the change of the arrangement of the plurality ofradiation detecting elements.
 4. The sensitivity correction methodaccording to claim 3, wherein when a different calibration radiationsource from a first calibration radiation source is set to a secondcalibration radiation source after a predetermined calibration radiationsource is used as the first calibration radiation source before thechange of the arrangement of the plurality of radiation detectingelements, a correction coefficient is obtained from a ratio ofcoincidence counting data collected at the same time using the samesecond calibration radiation source before and after the change of thearrangement of the plurality of radiation detecting elements, and thecorrection coefficient is multiplied by an expression represented by aproduct of respective factors related to sensitivity correction in thefirst calibration radiation source.
 5. The sensitivity correction methodaccording to claim 1, wherein coincidence counting data collected afterthe change of the arrangement of the plurality of radiation detectingelements corresponds to a combination geometrically affected by at leasta positional relation of the radiation detecting elements.
 6. Thesensitivity correction method according to claim 1, wherein sensitivitycoefficients classified according to sensitivity factors are obtainedbased on coincidence counting data collected by detecting radiation froma rod-shaped radiation source for calibration rotatable along theplurality of radiation detecting elements, a ring radiation source forcalibration provided along the plurality of radiation detectingelements, or a plate radiation source for calibration rotatable alongthe plurality of radiation detecting elements in the first sensitivitycoefficient calculation step, and a sensitivity coefficient of anon-geometric factor is obtained based on coincidence counting datacollected by detecting radiation from a cylindrical radiation source forcalibration before the change of the arrangement of the plurality ofradiation detecting elements in the second sensitivity coefficientcalculation step.
 7. The sensitivity correction method according toclaim 1, wherein solid angle correction for correcting a detectordistance is performed by dividing a crystal interference factor of aline of response (LOR) corresponding to a virtual straight lineconnecting two coinciding detectors by a square of a detector distanceof the LOR.
 8. The sensitivity correction method according to claim 1,wherein a plurality of detector heads, each of which has the pluralityof radiation detecting elements, is included, and the respectivedetector heads are configured such that a distance between therespective detector heads is changeable, a plate radiation source forcalibration installed at a center of the respective detector heads or arod-shaped radiation source for calibration allowed to reciprocate alonga central axis of the respective detector heads is used, and asensitivity coefficient derived from geometrical arrangement is obtainedbased on collected coincidence counting data by detecting radiation fromthe plate radiation source for calibration or the reciprocatingrod-shaped radiation source for calibration in a state in which therespective detector heads are brought close to each other in the thirdsensitivity coefficient calculation step.
 9. A radiation tomographydevice performing radiation tomography by detecting radiation generatedfrom a radiopharmaceutical in a test object, the radiation tomographydevice comprising: a plurality of detector heads, each of which has aplurality of radiation detecting elements, the respective detector headsbeing configured such that a distance between the respective detectorheads is changeable; and arithmetic means that performs an arithmeticoperation, wherein the arithmetic means performs a first sensitivitycoefficient calculation step of obtaining sensitivity coefficientsclassified according to sensitivity factors based on coincidencecounting data collected by the plurality of radiation detectingelements, a second sensitivity coefficient calculation step of obtaininga sensitivity coefficient of a non-geometric factor based on thecollected coincidence counting data before change of arrangement of theplurality of radiation detecting elements after the first sensitivitycoefficient calculation step, and a third sensitivity coefficientcalculation step of obtaining a sensitivity coefficient derived fromgeometrical arrangement based on the collected coincidence counting datain a state in which the arrangement of the plurality of radiationdetecting elements is changed after the second sensitivity coefficientcalculation step, and sensitivities of the radiation detecting elementsare corrected using the sensitivity coefficients obtained in the firstsensitivity coefficient calculation step, the second sensitivitycoefficient calculation step, and the third sensitivity coefficientcalculation step.
 10. The radiation tomography device according to claim9, wherein the detector heads are configured by disposing the respectiveradiation detecting elements in an arcuate shape.